Method of preparing a monosize silver halide emulsion involving Ostwald ripening followed by a crystal growth stage

Abstract

A method of preparing a monosize silver halide emulsion of which most of the silver halide crystals are of the twinned octahedral type is provided. An aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium halide in an aqueous dispersing medium containing a protective colloid are mixed at such concentration that the silver halide is nucleated, causing the silver halide nuclei to increase in size in the presence of a silver halide solvent by Ostwald ripening to produce a population of twinned octahedral seed crystals. Then the maximum rate of addition of silver salt and alkali metal or ammonium halide which is possible without renucleation occurring and the minimum rate of addition of silver salt and alkali metal or ammonium halide which is possible without Ostwald ripening occurring is determined and the seed crystals are caused to grow by adding to the aqueous dispersing medium aqueous silver salt solution and aqueous alkali metal or ammonium halide solution at a rate between the predetermined maximum and minimum rates and then at intervals the maximum and minimum addition rates are re-determined and the addition rates of silver and halide are adjusted to ensure that at all times during the crystal growth stage neither renucleation nor Ostwald ripening occurs, the p Br of the liquid phase being maintained above 0.15 during the whole crystal growth stage. The silver halide emulsions are useful for preparing photographic material.

I claim: 
     
       1. A method of preparing a monosize silver halide emulsion of which most of the silver halide crystals are of the twinned octahedral type which comprises a. mixing an aqueous solution of a silver salt and an aqueous solution of an alkali metal or ammonium halide in an aqueous dispersing medium containing a protective colloid at such a concentration that the silver halide is nucleated,   b. causing the silver halide nuclei to increase in size in the presence of a silver halide solvent by Ostwald ripening to produce a population of twinned octahedral seed crystals,   c. determining the maximum rate of addition of silver salt and alkali metal or ammonium halide which is possible without renucleation occurring and determining the minimum rate of addition of silver salt and alkali metal or ammonium halide which is possible without Ostwald ripening occurring, said determination is carried out by taking samples of the crystals, adding an equal quantity of silver halide at increasing and decreasing rates, respectively, and observing on a microscope when renucleation and Ostwald ripening, respectively, occur,   d. causing the seed crystals to grow by adding to the aqueous dispersing medium aqueous silver salt solution and aqueous alkali metal or ammonium halide solution at a rate between the predetermined maximum and minimum rates and then at intervals re-determining the maximum and minimum addition rates and adjusting the addition rates of silver and halides to ensure that at all times during the crystal growth stage neither renucleation nor Ostwald ripening occurs, the p Br of the liquid phase being maintained above 0.15 during the whole crystal growth stage.   
     
     
       2. A method according to claim 1 wherein during crystal growth the pBr of the liquid phase is controlled between 1.00 and 1.30 and the temperature of the liquid phase is between 45° to 55° C. 
     
     
       3. A method according to claim 2 wherein during crystal growth the pBr of the liquid phase is controlled at 1.15 and the temperature of the liquid phase is 50° C. 
     
     
       4. A method according to claim 1 wherein the formation of twinned seed crystals takes place at a pAg of over 11.0 and the growth of the twinned octahedral seed crystals takes place at a pAg below 10.1. 
     
     
       5. A method according to claim 1 wherein the aqueous silver salt solution and the aqueous alkali metal or ammonium halide solution are added at a continuously increasing rate which is approximately proportional to the time from the start of crystal growth after seed formation, the remainder of the conditions in the dispersing medium which affect the solubility of the silver halide in solution being maintained constant. 
     
     
       6. A method according to claim 1 wherein the rate of addition of the silver salt and the alkali metal or ammonium halide during crystal growth is ninety percent of the maximum rate of addition which is possible without renucleation occuring. 
     
     
       7. A method according to claim 1 wherein the aqueous silver salt solution and the aqueous alkali-metal or ammonium halide solution are added in step-wise increased amounts, the rates of which are approximately proportional to the time from the start of crystal growth after the formation of the twinned octahedral seed crystals, the remainder of the conditions in the dispersing medium which effect the solubility of the silver halide in solution being maintained constant. 
     
     
       8. A monosized silver halide emulsion which has been prepared by the method claimed in claim 1. 
     
     
       9. Photographic material which comprises in a layer thereof at least one emulsion as claimed in claim 8.